Within modern communication technology, signals are often sent over optical conduction paths, for example in optical fibres. A communication system normally comprises different interfaces for influencing, converting or amplifying the signals that are sent along the conduction path. Such interfaces may for example be arranged between different kinds of fibres, between a line for electrical signals and a line for optical signals, between fibres for different optical wavelengths etc. Interfaces of such and similar kinds may often with a common name be called “media converters”. A media converter that transmits optical signals to an optical conduction path normally includes an optical emitter that receives electrical signals on an input side and that emits these signals as optical signals.
For example EP-A-403455 describes an arrangement that comprises an interface that receives signals from a multi-mode fibre, that converts these signals to electrical signals and that with the help of a transmitter, transmits optical signals on a single mode fibre.
In a communication system, signals may often be transmitted with different speed of transmission. Different speeds of transmission means that different amounts of information may be transmitted per time unit. The speed of transmission is often measured in the number of bits per second. It is often desirable to supervise or limit the speed of transmission along a communication path. For example, it may be the case that a telecommunication or computer operator allows a client to send data over one or more optical fibres that are owned by the operator and that thereby the client pays for being able to transmit data with a certain bit rate, which corresponds to a certain amount of information per time unit. A problem thereby is that the client could increase the speed of transmission, sometimes without self being aware thereof, without the operator being informed about this fact.
It is known that it is possible to limit the speed of transmission in a communication system by means of a low pass filter, i.e. a filter that is arranged on an electric conduction path and that controls with how high frequencies signals may be transmitted on the conduction path. The above mentioned EP-A-403455 describes that the highest possible speed of transmission is controlled by means of a filter that limits the bandwidth of the transmitted signals. The document also mentions that the bandwidth of the filter may be controlled from a centre that supervises the communication system.
The use of such filters involves several problems. It is difficult to produce a filter, the bandwidth of which may be controlled from the outside and over a large frequency interval. Because of this difficulty, it is common that such filters may only be turned on or off. The production of controllable filters is expensive and complicated. Such filters do not give any sharp limit between where transmission functions and where transmission does not function. The transmission may thus in certain cases function if and only if the transmitted signals are powerful enough. The filter may thus impair the possibility to transmit signals of a high frequency (a high bit rate), but the transmission of such frequencies may still be possible under favourable circumstances, for example over a short fibre with a low attenuation.